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<refentry id="yasm_arch">

 <refentryinfo>
  <title>Yasm Supported Architectures</title>
  <date>October 2006</date>
  <productname>Yasm</productname>
  <author>
   <firstname>Peter</firstname>
   <surname>Johnson</surname>
   <affiliation>
    <address><email>peter@tortall.net</email></address>
   </affiliation>
  </author>

  <copyright>
   <year>2004</year>
   <year>2005</year>
   <year>2006</year>
   <year>2007</year>
   <holder>Peter Johnson</holder>
  </copyright>
 </refentryinfo>

 <refmeta>
  <refentrytitle>yasm_arch</refentrytitle>
  <manvolnum>7</manvolnum>
 </refmeta>

 <refnamediv>
  <refname>yasm_arch</refname>
  <refpurpose>Yasm Supported Target Architectures</refpurpose>
 </refnamediv>

 <refsynopsisdiv>
  <cmdsynopsis>
   <command>yasm</command>
   <arg choice="plain">
    <option>-a <replaceable>arch</replaceable></option>
   </arg>
   <arg choice="opt">
    <option>-m <replaceable>machine</replaceable></option>
   </arg>
   <arg choice="plain">
    <option><replaceable>...</replaceable></option>
   </arg>
  </cmdsynopsis>
 </refsynopsisdiv>

 <refsect1>
  <title>Description</title>

  <para>The standard Yasm distribution includes a number of modules
   for different target architectures.  Each target architecture can
   support one or more machine architectures.</para>

  <para>The architecture and machine are selected on the
   
   <citerefentry>
    <refentrytitle>yasm</refentrytitle>
    <manvolnum>1</manvolnum>
   </citerefentry>
   
   command line by use of the <option>-a
    <replaceable>arch</replaceable></option> and <option>-m
    <replaceable>machine</replaceable></option> command line options,
   respectively.</para>

  <para>The machine architecture may also automatically be selected by
   certain object formats.  For example, the <quote>elf32</quote>
   object format selects the <quote>x86</quote> machine architecture
   by default, while the <quote>elf64</quote> object format selects
   the <quote>amd64</quote> machine architecture by default.</para>
 </refsect1>

 <refsect1>
  <title>x86 Architecture</title>

  <para>The <quote>x86</quote> architecture supports the IA-32
   instruction set and derivatives and the AMD64 instruction set.  It
   consists of two machines: <quote>x86</quote> (for the IA-32 and
   derivatives) and <quote>amd64</quote> (for the AMD64 and
   derivatives).  The default machine for the <quote>x86</quote>
   architecture is the <quote>x86</quote> machine.</para>

  <refsect2>
   <title>BITS Setting</title>

   <para>The x86 architecture BITS setting specifies to Yasm the
    processor mode in which the generated code is intended to execute.
    x86 processors can run in three different major execution modes:
    16-bit, 32-bit, and on AMD64-supporting processors, 64-bit.  As
    the x86 instruction set contains portions whose function is
    execution-mode dependent (such as operand-size and address-size
    override prefixes), Yasm cannot assemble x86 instructions
    correctly unless it is told by the user in what processor mode the
    code will execute.</para>

   <para>The BITS setting can be changed in a variety of ways.  When
    using the NASM-compatible parser, the BITS setting can be changed
    directly via the use of the <userinput>BITS xx</userinput>
    assembler directive.  The default BITS setting is determined by
    the object format in use.</para>
  </refsect2>

  <refsect2>
   <title>BITS 64 Extensions</title>

   <para>The AMD64 architecture is a new 64-bit architecture developed
    by AMD, based on the 32-bit x86 architecture. It extends the
    original x86 architecture by doubling the number of general
    purpose and SIMD registers, extending the arithmetic operations
    and address space to 64 bits, as well as other features.</para>

   <para>Recently, Intel has introduced an essentially identical
    version of AMD64 called EM64T.</para>

   <para>When an AMD64-supporting processor is executing in 64-bit
    mode, a number of additional extensions are available, including
    extra general purpose registers, extra SSE2 registers, and
    RIP-relative addressing.</para>

   <para>Yasm extends the base NASM syntax to support AMD64 as
    follows.  To enable assembly of instructions for the 64-bit mode
    of AMD64 processors, use the directive <userinput>BITS
     64</userinput>. As with NASM's BITS directive, this does not
    change the format of the output object file to 64 bits; it only
    changes the assembler mode to assume that the instructions being
    assembled will be run in 64-bit mode.  To specify an AMD64 object
    file, use <option>-m amd64</option> on the Yasm command line, or
    explicitly target a 64-bit object format such as <option>-f
     win64</option> or <option>-f elf64</option>.</para>

   <refsect3>
    <title>Register Changes</title>

    <para>The additional 64-bit general purpose registers are named
     r8-r15.  There are also 8-bit (rXb), 16-bit (rXw), and 32-bit
     (rXd) subregisters that map to the least significant 8, 16, or 32
     bits of the 64-bit register.  The original 8 general purpose
     registers have also been extended to 64-bits: eax, edx, ecx, ebx,
     esi, edi, esp, and ebp have new 64-bit versions called rax, rdx,
     rcx, rbx, rsi, rdi, rsp, and rbp respectively.  The old 32-bit
     registers map to the least significant bits of the new 64-bit
     registers.</para>

    <para>New 8-bit registers are also available that map to the 8
     least significant bits of rsi, rdi, rsp, and rbp.  These are
     called sil, dil, spl, and bpl respectively.  Unfortunately, due
     to the way instructions are encoded, these new 8-bit registers
     are encoded the same as the old 8-bit registers ah, dh, ch, and
     bh.  The processor tells which is being used by the presence of
     the new REX prefix that is used to specify the other extended
     registers.  This means it is illegal to mix the use of ah, dh,
     ch, and bh with an instruction that requires the REX prefix for
     other reasons.  For instance:</para>

    <screen>add ah, [r10]</screen>
                
    <para>(NASM syntax) is not a legal instruction because the use of
     r10 requires a REX prefix, making it impossible to use ah.</para>

    <para>In 64-bit mode, an additional 8 SSE2 registers are also
     available.  These are named xmm8-xmm15.</para>
   </refsect3>

   <refsect3>
    <title>64 Bit Instructions</title>

    <para>By default, most operations in 64-bit mode remain 32-bit;
     operations that are 64-bit usually require a REX prefix (one bit
     in the REX prefix determines whether an operation is 64-bit or
     32-bit).  Thus, essentially all 32-bit instructions have a 64-bit
     version, and the 64-bit versions of instructions can use extended
     registers <quote>for free</quote> (as the REX prefix is already
     present).  Examples in NASM syntax:</para>

    <screen>mov eax, 1  ; 32-bit instruction</screen>
    <screen>mov rcx, 1  ; 64-bit instruction</screen>

    <para>Instructions that modify the stack (push, pop, call, ret,
     enter, and leave) are implicitly 64-bit.  Their 32-bit
     counterparts are not available, but their 16-bit counterparts
     are.  Examples in NASM syntax:</para>

    <screen>push eax  ; illegal instruction</screen>
    <screen>push rbx  ; 1-byte instruction</screen>
    <screen>push r11  ; 2-byte instruction with REX prefix</screen>
   </refsect3>

   <refsect3>
    <title>Implicit Zero Extension</title>

    <para>Results of 32-bit operations are implicitly zero-extended to
     the upper 32 bits of the corresponding 64-bit register.  16 and 8
     bit operations, on the other hand, do not affect upper bits of
     the register (just as in 32-bit and 16-bit modes).  This can be
     used to generate smaller code in some instances.  Examples in
     NASM syntax:</para>

    <screen>mov ecx, 1  ; 1 byte shorter than mov rcx, 1</screen>
    <screen>and edx, 3  ; equivalent to and rdx, 3</screen>
   </refsect3>

   <refsect3>
    <title>Immediates</title>

    <para>For most instructions in 64-bit mode, immediate values
     remain 32 bits; their value is sign-extended into the upper 32
     bits of the target register prior to being used.  The exception
     is the mov instruction, which can take a 64-bit immediate when
     the destination is a 64-bit register.  Examples in NASM
     syntax:</para>

    <screen>add rax, 1           ; optimized down to signed 8-bit</screen>
    <screen>add rax, dword 1     ; force size to 32-bit</screen>
    <screen>add rax, 0xffffffff  ; sign-extended 32-bit</screen>
    <screen>add rax, -1          ; same as above</screen>
    <screen>add rax, 0xffffffffffffffff ; truncated to 32-bit (warning)</screen>
    <screen>mov eax, 1           ; 5 byte</screen>
    <screen>mov rax, 1           ; 5 byte (optimized to signed 32-bit)</screen>
    <screen>mov rax, qword 1     ; 10 byte (forced 64-bit)</screen>
    <screen>mov rbx, 0x1234567890abcdef ; 10 byte</screen>
    <screen>mov rcx, 0xffffffff  ; 10 byte (does not fit in signed 32-bit)</screen>
    <screen>mov ecx, -1          ; 5 byte, equivalent to above</screen>
    <screen>mov rcx, sym         ; 5 byte, 32-bit size default for symbols</screen>
    <screen>mov rcx, qword sym   ; 10 byte, override default size</screen>

    <para>The handling of mov reg64, unsized immediate is different
     between YASM and NASM 2.x; YASM follows the above behavior, while
     NASM 2.x does the following:</para>

    <screen>add rax, 0xffffffff  ; sign-extended 32-bit immediate</screen>
    <screen>add rax, -1          ; same as above</screen>
    <screen>add rax, 0xffffffffffffffff ; truncated 32-bit (warning)</screen>
    <screen>add rax, sym         ; sign-extended 32-bit immediate</screen>
    <screen>mov eax, 1           ; 5 byte (32-bit immediate)</screen>
    <screen>mov rax, 1           ; 10 byte (64-bit immediate)</screen>
    <screen>mov rbx, 0x1234567890abcdef ; 10 byte instruction</screen>
    <screen>mov rcx, 0xffffffff  ; 10 byte instruction</screen>
    <screen>mov ecx, -1          ; 5 byte, equivalent to above</screen>
    <screen>mov ecx, sym         ; 5 byte (32-bit immediate)</screen>
    <screen>mov rcx, sym         ; 10 byte instruction</screen>
    <screen>mov rcx, qword sym   ; 10 byte (64-bit immediate)</screen>
   </refsect3>

   <refsect3>
    <title>Displacements</title>

    <para>Just like immediates, displacements, for the most part,
     remain 32 bits and are sign extended prior to use.  Again, the
     exception is one restricted form of the mov instruction: between
     the al/ax/eax/rax register and a 64-bit absolute address (no
     registers allowed in the effective address).  In NASM syntax, use
     of the 64-bit absolute form requires
     <userinput>[qword]</userinput>.  Examples in NASM syntax:</para>

    <screen>mov eax, [1]    ; 32 bit, with sign extension</screen>
    <screen>mov al, [rax-1] ; 32 bit, with sign extension</screen>
    <screen>mov al, [qword 0x1122334455667788] ; 64-bit absolute</screen>
    <screen>mov al, [0x1122334455667788] ; truncated to 32-bit (warning)</screen>
   </refsect3>

   <refsect3>
    <title>RIP Relative Addressing</title>

    <para>In 64-bit mode, a new form of effective addressing is
     available to make it easier to write position-independent code.
     Any memory reference may be made RIP relative (RIP is the
     instruction pointer register, which contains the address of the
     location immediately following the current instruction).</para>

    <para>In NASM syntax, there are two ways to specify RIP-relative
     addressing:</para>

    <screen>mov dword [rip+10], 1</screen>

    <para>stores the value 1 ten bytes after the end of the
     instruction.  <userinput>10</userinput> can also be a symbolic
     constant, and will be treated the same way.  On the other
     hand,</para>

    <screen>mov dword [symb wrt rip], 1</screen>

    <para>stores the value 1 into the address of symbol
     <userinput>symb</userinput>.  This is distinctly different than
     the behavior of:</para>

    <screen>mov dword [symb+rip], 1</screen>

    <para>which takes the address of the end of the instruction, adds
     the address of <userinput>symb</userinput> to it, then stores the
     value 1 there.  If <userinput>symb</userinput> is a variable,
     this will <emphasis>not</emphasis> store the value 1 into the
     <userinput>symb</userinput> variable!</para>

    <para>Yasm also supports the following syntax for RIP-relative
     addressing:</para>

    <screen>mov [rel sym], rax  ; RIP-relative</screen>
    <screen>mov [abs sym], rax  ; not RIP-relative</screen>

    <para>The behavior of:</para>

    <screen>mov [sym], rax</screen>

    <para>Depends on a mode set by the DEFAULT directive, as follows.
     The default mode is always "abs", and in "rel" mode, use of
     registers, an fs or gs segment override, or an explicit "abs"
     override will result in a non-RIP-relative effective
     address.</para>
  
    <screen>default rel</screen>
    <screen>mov [sym], rbx      ; RIP-relative</screen>
    <screen>mov [abs sym], rbx  ; not RIP-relative (explicit override)</screen>
    <screen>mov [rbx+1], rbx    ; not RIP-relative (register use)</screen>
    <screen>mov [fs:sym], rbx   ; not RIP-relative (fs or gs use)</screen>
    <screen>mov [ds:sym], rbx   ; RIP-relative (segment, but not fs or gs)</screen>
    <screen>mov [rel sym], rbx  ; RIP-relative (redundant override)</screen>

    <screen>default abs</screen>
    <screen>mov [sym], rbx      ; not RIP-relative</screen>
    <screen>mov [abs sym], rbx  ; not RIP-relative</screen>
    <screen>mov [rbx+1], rbx    ; not RIP-relative</screen>
    <screen>mov [fs:sym], rbx   ; not RIP-relative</screen>
    <screen>mov [ds:sym], rbx   ; not RIP-relative</screen>
    <screen>mov [rel sym], rbx  ; RIP-relative (explicit override)</screen>
   </refsect3>

   <refsect3>
    <title>Memory references</title>

    <para>Usually the size of a memory reference can be deduced by
     which registers you're moving--for example, "mov [rax],ecx" is a
     32-bit move, because ecx is 32 bits.  YASM currently gives the
     non-obvious "invalid combination of opcode and operands" error if
     it can't figure out how much memory you're moving.  The fix in
     this case is to add a memory size specifier: qword, dword, word,
     or byte.</para>

    <para>Here's a 64-bit memory move, which sets 8 bytes starting at
     rax:</para>

    <screen>mov qword [rax], 1</screen>

    <para>Here's a 32-bit memory move, which sets 4 bytes:</para>

    <screen>mov dword [rax], 1</screen>

    <para>Here's a 16-bit memory move, which sets 2 bytes:</para>

    <screen>mov word [rax], 1</screen>

    <para>Here's an 8-bit memory move, which sets 1 byte:</para>

    <screen>mov byte [rax], 1</screen>
   </refsect3>
  </refsect2>
 </refsect1>

 <refsect1>
  <title>lc3b Architecture</title>

  <para>The <quote>lc3b</quote> architecture supports the LC-3b ISA as
   used in the ECE 312 (now ECE 411) course at the University of
   Illinois, Urbana-Champaign, as well as other university courses.
   See <ulink url="http://courses.ece.uiuc.edu/ece411/"/> for more
   details and example code.  The <quote>lc3b</quote> architecture
   consists of only one machine: <quote>lc3b</quote>.</para>
 </refsect1>

 <refsect1>
  <title>See Also</title>

  <para><citerefentry>
   <refentrytitle>yasm</refentrytitle>
   <manvolnum>1</manvolnum>
  </citerefentry></para>
 </refsect1>

 <refsect1>
  <title>Bugs</title>

  <para>When using the <quote>x86</quote> architecture, it is overly
   easy to generate AMD64 code (using the <userinput>BITS
    64</userinput> directive) and generate a 32-bit object file (by
   failing to specify <option>-m amd64</option> on the command line or
   selecting a 64-bit object format).  Similarly, specifying
   <option>-m amd64</option> does not default the BITS setting to
   64.  An easy way to avoid this is by directly specifying
   a 64-bit object format such as <option>-f elf64</option>.</para>
 </refsect1>
</refentry>
